1. FIELD OF THE INVENTION
This invention relates to a MOS-type semiconductor device and a method of making the same, and more particularly to a MOS-type semiconductor device using a gate insulating film including a silicon nitride film and a method of making the same.
2. DESCRIPTION OF THE RELATED ART
Partly because of the stability of the film itself in the manufacturing process and partly because of the excellent electrical insulation property, an SiO.sub.2 film has been used as the gate insulating film for MOS-type transistors.
However, with the recent progressive microminiaturization of transistors, the problem of resistance against hot carriers has manifested itself. More specifically, the Si--O bond is broken by the hot carriers diffused into the SiO film and an interface level or a trap level is generated in the film, which are the causes of reducing the lifetime of the transistors. In addition, there is another problem that when an SiO.sub.2 is formed, the SiO.sub.2 entraps vacancies or metal impurities (Fe or Cu) in the semiconductor substrate, thus decreasing the dielectric strength of the gate insulating film, resulting in a reduction of the insulation property.
For this reason, as the gate insulating film of the MOS transistor, it has been proposed to use an ONO film of a three-layer structure including an oxide film (SiO.sub.2 silicon dioxide), a nitride film (Si.sub.3 N.sub.4 silicon nitride), and another oxide film (SiO.sub.2 silicon dioxide). (As an example, refer to T. Hori et al.: "Deep-Submicrometer CMOS Technology with Reoxidized or Annealed Nitrided-Oxide Gate Dielectrics Prepared by Rapid Thermal Processing" IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. 39, No. 1, JANUARY 1992, pp. 118-126).
FIG. 1 shows a schematic construction of a MOS transistor using an ONO film as the gate insulating film. To produce this MOS transistor 11 by the method in the above paper, a SiO.sub.2 film 13 is formed on the surface of a Si substrate 12 by thermal oxidation process, a Si.sub.3 N.sub.4 film 14 is formed by rapid thermal nitridation on the surface of the SiO.sub.2 film by RTP (Rapid Thermal Processing), and a SiO.sub.2 film 15 is formed by re-oxidizing the surface of the Si.sub.3 N.sub.4.
Those films 13 to 15 constitute an ONO film 16 to serve as the gate insulating film, and on the ONO film 16, a polycrystalline Si film 17 is formed in a pattern of the gate electrode, and diffused layers 18 of the source and the drain are formed in the Si substrate at both sides of the polycrystalline Si film 17.
Since the bond energy of the Si--n bond in the silicon nitride film is greater than that of Si--O bond, the ONO film 16 of the MOS transistor thus produced is more excellent in the resistance against hot carriers and the TDDB (Time Dependent Dielectric Breakdown) characteristics than the SiO.sub.2 film alone.
In order to suppress the short channel effect derived from microminiaturization, it becomes more frequent to introduce P-type impurities, in place of N-type impurities, into the polycrystalline Si film serving as the gate electrode of P-channel MOS transistors. In this case, the gate insulating film using the ONO film 16 is effective to suppress the boron used as the P-type impurities penetrating through the gate insulating film.
As described above, when a gate-insulating ONO film is used, the ability of suppressing the impurity diffusion is improved and, therefore, the effect of suppressing the impurities punching through the gate insulating film from the gate electrode is enhanced. It has been reported that in a MOS transistor using a nitrided, oxidized film formed (reoxidation) by RTP, electron trapping is also suppressed by the reoxidation, and additionally, the hot carrier reliability is improved better than in the case where the gate insulating film is formed only by an oxidized film.
(T. Hori: "Improvement in Gate Insulating Film Characteristics by Nitride and Oxide Film Prepared by Rapid Thermal Processing", Applied Physics, Vol. 60, No. 11 (1991), pp. 1127-1130).
However, in a MOS transistor using an ONO film, there is a problem that the peak values of the transconductance indicative of the current drive capacity of the transistor become lower than in the case where the gate insulating film is formed only by an oxide film.
FIGS. 2A and 2B show the Vg-Vt dependence of the transconductance of the 10-.mu.m-channel and 0.3-.mu.m-channel transistors using ONO film and pure oxide film as the gate insulating films, respectively when the drain-source voltage Vd=2 V, where Vg is the gate voltage and Vt is the threshold voltage. One can find from FIGS. 2A and 2B that the peak values of the transconductance of the transistors using the ONO film are lower than those of the transistors using the pure oxide film.